1. Field of the Invention
The invention relates to an image forming apparatus adapted to again convey a sheet having an image formed on the front surface (one surface) thereof to an image forming portion and form an image, and particularly to the construction of a re-conveying path for conveying a sheet again to the image forming portion.
2. Related Background Art
Heretofore, some of image forming apparatus such as printers having a digital optical system have been adapted to again convey a sheet having an image formed on the front side (one side) thereof to an image forming portion and form an image, and such an image forming apparatus is provided with a re-conveying path for again conveying the sheet having an image formed on the front side (one side) thereof to the image forming portion.
In such an image forming apparatus, as described, for example, in Japanese Patent Application Laid-open No. H07-267461, design is made such that a sheet contained in a sheet supplying cassette is fed to a sheet feeding apparatus, whereafter the fed sheet is conveyed to an image forming portion, a fixing portion and a discharging portion in succession by a sheet conveying apparatus, and when images are to be formed on the two sides of the sheet, the sheet is conveyed to a re-conveying path and is again conveyed to the image forming portion.
Now, as such a sheet conveying apparatus, use is widely been made of one provided with sheet conveying portions for conveying a sheet while nipping it between a pair of rotating cylindrical rollers, between a cylindrical roller and a runner, and between a cylindrical roller and a pad or the like, and these sheet conveying portions are disposed in a sheet conveying path such as a re-conveying path to thereby convey the sheet. The rotating speeds and ON-OFF timing of these sheet conveying portions are changed, whereby it becomes possible to control the movement of the sheet easily.
Now, adjacent ones of the sheet conveying portions, even in the most spaced apart state, are disposed with an interval which enables a sheet shortest in the length in a conveying direction (hereinafter referred to as the shortest sheet) to be nipped. Also, the sheet conveying path is curved for the downsizing of the apparatus and the construction of each processing, and in a case where the sheet conveying portions are provided in such a curved sheet conveying path, the sheet, from after it is conveyed from the upstream sheet conveying portion until it is nipped by the downstream sheet conveying portion, is conveyed along a sheet conveying guide portion or the like provided between the two sheet conveying portions.
FIG. 5 of the accompanying drawings schematically shows the construction of a conventional image forming apparatus provided with such a sheet conveying apparatus, and a sheet P discharged from a sheet supplying cassette 50 containing sheets P therein by a feeding apparatus 51 is first conveyed to an image forming portion 60 by sheet conveying portions 52 and 53 disposed downstream of the feeding apparatus 51 and at positions whereat they can nip the shortest sheet at a time, and a toner image formed in this image forming portion 60 is transferred to the sheet P.
Next, the sheet P to which the toner image has been transferred is conveyed to a fixing apparatus 61, where the toner image is fixed, whereafter the sheet P is discharged to a discharged sheet stacking portion 72 by discharging rollers 70 via a discharged sheet conveying route 71.
Now, this image forming apparatus is designed to be capable of effecting two-side printing which forms images on the two-sides of the sheet P, and multiplex printing which forms images on the sheet P in superimposed relationship with one another, and design is made such that when the two-side printing is to be effected, or when the multiplex printing is to be effected, the sheet having had an image formed on one side thereof after it has been passed through the fixing apparatus 61 is partly discharged to the discharged sheet stacking portion 72, whereafter the sheet is directed to a reversal conveying path 80 which is a re-conveying path by the reverse rotation of the discharging rollers 70 and a changeover mechanism (not shown), and is again conveyed to the image forming portion 60 via the reversal conveying path 80.
Then, the sheet P thus conveyed to the image forming portion 60 receives the transfer of a toner image in the image forming portion 60 as in the case of one-side printing, whereafter the toner image thereon is fixed in the fixing apparatus 61, whereafter the sheet P is discharged to the discharged sheet stacking portion 72 via the discharged sheet conveying route 71.
In such a conventional image forming apparatus, however, if as shown in FIG. 5, the reversal conveying path 80 is provided in a vertical direction, it is necessary that for example, a U-shaped reversing portion 81 for changing the conveying direction of the downwardly conveyed sheet P and directing it to the image forming portion 60 be provided downstream of the reversal conveying path 80. When the U-shaped reversing portion 81 is thus provided downstream of the reversal conveying path 80, the height of the apparatus body of the apparatus becomes greater by an amount corresponding to the vertical length of the reversing portion 81, and the size of the entire apparatus also becomes larger.